Exporting OBJ missing map_Kd
In the export window verify the checkbox "Write materials" is checked. Also ensure you have all the textures as external files (not packed in the blend file itself).
Also ensure you have created the material. In Blender, it's possible to see a textured object in the viewport after unwrapping it, but without creating a material at all. If the material isn't created, you might see it in the viewport, but it won't be "real." =)